Conventional optical fibers comprise generally a cladding, usually of quartz (silica) glass, which is protected mechanically by a layer of a plastic forming a coating or jacket, most often of polyacrylate plastic, having a typical thickness of 60-65 .mu.m. For monitoring the concentricity of such protective coatings in the manufacture of an optical fiber most frequently an optical method is used, which uses scattering patterns, fringes, in the forward direction. However, this method does not work for fibers, which are coated with a thin layer of polyimide having a thickness of the order of magnitude of 20 .mu.m. This results from the different optical characteristics of such a thin polyimide layer, partly derived from the reduced thickness thereof, partly from the fact that polyimide materials have substantially larger refractive indices, compared to the conventional polyacrylate materials.
In U.S. Pat. No. 5,208,645, which corresponds to the published European patent application EP-A2 0 443 322, a method and a device are disclosed for optical detection of the thickness of the coatings of cylindrical objects, such as coatings on optical fibers. A fiber is irradiated with light, the intensity or energy of which is measured after a reflection by means of a photodetector. The intensity is a measure of the thickness of the coating. This document is especially focused on measurements on optical fibers coated with carbon. Two principles are described. In one, the fiber is illuminated with parallel beams orthogonally in relation to the longitudinal direction of the fiber, the reflected radiation is detected with an image detector and the thickness of the coating is finally determined from the peak levels of the intensity of light condensed by the lens effect of the fiber portion (column 2, lines 22-34). In the other method the intensity of reflected light is measured and compared to a calibration curve in order to obtain a measure of the thickness of the coating (column 3, lines 31-51 and the description of FIGS. 4a-4c).
In the published Japanese patent application JP-A 4-363612 methods similar to that described in the document discussed above are disclosed. The thickness of a coating of an optical fiber is determined by illuminating the fiber from the side and then measuring the intensity of light, which has passed through the fiber. Possibly the intensity of reflected light is also measured. The thickness of the coating is then determined by comparing the intensity of the light that has passed through, or possibly the intensity of the reflected light, to a calibration curve.
In determining the position of the coating according to U.S. Pat. No. 4,583,851 a fiber is illuminated from the side thereof and the reflected light intensity is measured in relation to the output angle. Any deviation from the symmetry of a given profile indicates an eccentric position. A light source 10 illuminates an optical fiber 12 and the intensity of the reflected light as a function of the detection angle is detected by means of a converging lens 14, an rotating mirror 16, a slit aperture 20 and a photodetector 18.
In the published Japanese patent application JP-A 63-274804 determination of the diameter of the core and the thickness of the coating of an optical fiber are determined by projecting two light beams having different wave lengths at right angles towards and through a fiber and measuring the absorption of the light beams. The principle is that one can get an estimation of the diameter of the core and the thickness of the coating from knowledge of the different absorption coefficients of the core and the coating and the measured intensity of the light which has been reflected and the measured intensity of the light which has passed through the fiber respectively. The method disclosed in the published Japanese patent application JP-A 15 63-286738 has a great similarity to this measurement procedure. The large difference seems to be that here only one light beam is used instead of two beams.
In U.S. Pat. No. 5,216,486 a device for detecting diameter deviations of preferably optical fibers is disclosed. A monochromatic light source is used and by means of lenses and prisms the light is divided in two beams which are detected by detectors D1 and D2. They generate two voltages V1 and V2, which are subtracted from each other. When the difference V1-V2 or V1-V2 is not equal to zero, an error has been detected (column 3, lines 50-62). In the published Japanese patent application JP-A 5-079821 (application No. 3-243284) measurement of the thickness of the coating on carbon coated optical fibers is described, in which the measurement uses probably absorption of a light beam passing through the fiber.
In the published Japanese patent application JP-A 5-288517 measurement of the thickness of a coating is described, in which an optical fiber, consisting of a glass portion 100a and a plastics coating 100b, is irradiated with either X-rays or ultra violet radiation, whereafter the intensity of light passing through the fiber is measured, processed and reproduced in order to form an estimate of the thickness of the coating (see FIGS. 2 and 3). The intensity is said to depend generally only on the absorption of the fiber core and the coating and not be materially affected by refraction.
In U.S. Pat. No. 3,017,512 determination of the thickness of a thin film on an object is described. Here a reference beam of infrared light having a wave length which is not absorbed by the coating and a test beam of infrared light having a wave length, which can be partly absorbed by the coating, are used. According to the formula in column 2 it is possible to calculate, having knowledge of the intensity of the mentioned beams after reflection, the thickness of the coating of the film. A measurement device is shown in FIG. 1, in which polychromatic infrared light is generated at 11, the light being divided, by means of a chopper 12 (see FIG. 2), mirrors and optical filters 28 and 29, in a reference ray 20 and a measurement ray 21, the intensities of which are measured by means of a detector 31 and amplified. In FIG. 3 another embodiment is shown, in which separate light sources 40 and 41 are provided making the use of a chopper 12 not necessary.